My significant others grandmother on his fathers side is Cherokee from OK how can we prove this

Dear Christine,
Thank you for your question, I am always glad to hear when people want to explore their Native identity and heritage. Native American cultures are diverse and beautiful. This exploration should be conducted with respect to our ancestors, past present and future. Explore away! We welcome you with open arms
Last fall I answered a similar question that you might find helpful. Check on WeRNative’s website here.
Good luck on your journey.
Auntie Manda